This patch is not really needed, because zlib is already used from system, but
it makes it easier to ensure that nothing else is bundled

diff --git a/src/external/CMakeLists.txt b/src/external/CMakeLists.txt
index e5af410..6d7bf6b 100644
--- a/src/external/CMakeLists.txt
+++ b/src/external/CMakeLists.txt
@@ -36,6 +36,7 @@ target_link_libraries(glues_stel Qt${QT_VERSION_MAJOR}::Gui)
 set_target_properties(glues_stel PROPERTIES AUTOMOC 0)
 SET_TARGET_PROPERTIES(glues_stel PROPERTIES FOLDER "src/external")
 
+if(0)
 SET(zlib_SRCS
      zlib/adler32.c
      zlib/compress.c
@@ -69,6 +70,7 @@ target_include_directories(zlib_stel PUBLIC zlib)
 #turn off automoc, not needed here
 set_target_properties(zlib_stel PROPERTIES AUTOMOC 0)
 SET_TARGET_PROPERTIES(zlib_stel PROPERTIES FOLDER "src/external")
+endif()
 
 ################################# INDI ################################
 IF(USE_PLUGIN_TELESCOPECONTROL)